This invention relates to a compander circuit for echo cancellation in telecommunications apparatus, such as terminals, transmission systems, or switching equipment, having an input xxe2x80x21, at which a send signal sent from a near-end subscriber 1 to a far-end subscriber 2 is applied, an input xxe2x80x22, at which a receive signal zin(k) coming from the far-end subscriber 2 and a far-end echo, possibly consisting of a number of component echoes, are applied, an output yxe2x80x21, at which the receive signal zout(k) processed in the compander circuit is passed on to the near-end subscriber 1, an estimator module for estimating an echo coupling and a delay of a line echo, and a characteristic module with an associated multiplier for controlling the short-time output level of zout(k) as a function of the short-time input level of zin(k).
Such a compander circuit is disclosed, for example, in DE 37 24 346 A1.
When speech signals are transmitted over telecommunications lines, acoustic echoes may be caused at the xe2x80x9cnear endxe2x80x9d, i.e., at the sending user, by the direct sound transmission from the loudspeaker to the microphone of the terminal. Particularly if several terminals are located near each other, for example in an office or conference room with many telephone terminals, the near-end echo problem is aggravated considerably since each loudspeaker signal is coupled to each microphone. This is to be remedied by a multichannel echo canceling method and circuit proposed in EP 0 627 825 A2.
Aside from the near-end acoustic echo, however, a line echo, i.e., an electrical echo, which may consist of different numbers of component echoes, is caused by reflections of transmitted speech signals at different points in the transmission channel, for example at 2- to 4-wire hybrids in the terminal or at central offices and at interfaces between analog and digital networks. A distinction is generally made between so-called near-end echoes, which are reflected to the talker in a period up to 128 ms, and so-called far-end echoes, which return to the talker in a period up to 640 ms. Because of the round-trip delay time, echoes are particularly disconcerting to the talker as the latter has meanwhile continued talking when his own reflected speech signal returns to him. Delayed far-end echoes occur, for example, during intercontinental calls, i.e., calls transmitted via deep sea cables or satellites. Operators of corresponding telephone networks are therefore anxious to suppress all kinds of echoes, particularly far-end echoes with long delays, but also near-end echoes, which may be reflected with a higher intensity, as effectively as possible.
EP 0 792 029 A2, for example, proposes an echo canceler with an adaptive filter which comprises a coarse near-end speech detector and a more accurate near-end speech detector which are disposed at both sides of the echo canceler and thus can observe the incoming signal prior to and after the echo cancellation. In non-speech intervals of the near end, an adaptation to reflected echo values is to be effected.
DE 44 30 189 A1 proposes a low-cost method suitable for use under different acoustic conditions which uses an FIR filter whose filter coefficients are determined by the NMLS algorithm. For echo cancellation in a telecommunications network with a plurality of parallel channels, such an echo canceler with an adaptive FIR filter, which is implemented in software on a digital signal processor, is employed for each channel. The adaptive FIR filter is to produce a replica of the total echo to be expected, which may consist of a number of component echoes, and subtract this replica from the actual echo signal transmitted to the telecommunications terminal.
U.S. Pat. No. 5,148,426 shows an echo canceling arrangement in which a PCM multiplexer and network monitoring functions are integrated with the echo canceler in one package in order to reduce the amount of cabling and patching required and the errors associated therewith. The technical implementation of a single echo canceler which can serve a plurality of communications channels simultaneously is not shown. Only the spatially separated functions, particularly auxiliary functions for the classical performance monitoring functions, are combined with the echo canceler in one unit. The echo canceling unit proper handles only single channels by measuring the signal-to-noise ratio and the echo level of each channel. In this manner, monitoring of the quality of the individual channels, of DS1 level performance (24 64-kb/s channels), and of DS3 level performance (28 times 24 channels) is implemented in a device of bidirectional design.
U.S. Pat. No. 4,467,146 discloses an echo canceler with a digital transversal filter which produces a replica of the for-end line echo and subtracts this replica from the incoming signal. The proposed arrangement is to improve the echo cancellation for digital PCM multiplex transmission systems in which speech samples are encoded with a quasi-logarithmic characteristic. Since prior-art digital echo cancelers operate with uniformly quantized sampling characteristics (linear), samples formed with nonuniform characteristics (A-law or m-law) must first be converted back to uniformly quantized samples. Errors are caused since the sum signal was nonuniformly quantized at the exchange-side analog-to-digital converter. According to U.S. Pat. No. 4,467,146, the errors of the nonuniform quantization are to be calculated and corrected.
Aside from the above-described method of filtering out replicated echo signals, a further, easier-to-implement echo cancellation method is known, namely the so-called compander principle. The coupling and delay of a line echo are estimated in an estimator circuit, and if the short-time receive level is correspondingly reduced, the output level is reduced during non-speech intervals by means of a characteristic module in such a manner that the incoming line echo and any background noise are effectively suppressed. No general echo subtraction from the incoming signal is performed in the receive path, but only a xe2x80x9cmaskingxe2x80x9d of the echo in the non-speech intervals by a corresponding reduction of the short-time output level. While one of the two subscribers is talking, i.e., in the presence of a high input level, the output level is held in an upper range by a corresponding shift in the characteristic module, since the considerably weaker echo is then xe2x80x9chiddenxe2x80x9d under the high level of the speech signal and has no disturbing effect. This compander principle is described, for example, in the above-mentioned DE 37 24 346 A1.
The prior-art echo canceler systems with companders are only suited for providing an echo cancellation function to individual transmission channels. The present invention has for its object to improve an echo canceling compander circuit of the kind described at the beginning in such a manner that with a single compander circuit, a plurality of channels, preferably more than 2000 channels, can be handled simultaneously, with a minimum amount of additional circuitry being required to modify the compander circuit.
The invention is characterized in that at all inputs and outputs of the compander circuit, a plurality of N communications channels are applied and outputted, respectively, at an N-fold clock rate as compared with a single communications channel; that all memory units in the compander circuit are designed as multiplexer-buffer-demultiplexer (MBD) units; and that a modulo-N counter is provided which controls all MBD units in such a manner that in each of the MBD units, always the same memory cell assigned to a particular communications channel is addressed.